To demonstrate that the impedance ratio of ductus arteriosus and pulmonary artery could be used as a diagnostic test for preterm, term and post-term pregnancies.

Methods

We included 134 pregnant patients, 52 preterm, 65 term and 17 post-term pregnancies. Ductus arteriosus and pulmonary artery blood flow velocity waveforms were investigated in all pregnancies by pulsated-wave Doppler ultrasound.
The study was conducted in Perinatology Unit, Hospitalary City “Dr. Enrique Tejera”, Valencia, Carabobo State, Venezuela between August 1999 and January 2000.

Methodology

In a sagittal or axial plane we first obtained the pulsed Doppler wave of the ductus arteriosus flow and measured the acceleration time and ejection time in milliseconds obtaining an impedance index (TA/TE). The same measurement was done for the pulsed wave of the pulmonary artery. The ratio between these two values was counted, the ductus arteriosus impedance index was divided by impedance index of the pulmonary artery. The result was multiplied by 1000 and was called IRIDAPta/te. (Impedance Relative Index between Ductus Arteriosus and Pulmonary artery using acceleration time / ejection time) This procedure which uses a noninvasive tool, reveals a redirection of blood flow from the ductus arteriosus to the pulmonary area as the pregnancy progresses.
This method has been used in our perinatology unit for past eleven years. Since implementation of this method we have not performed amniocentesis to assess lung maturity.

Results

The values of relative impedance index between ductus arteriosus and pulmonary artery in the preterm pregnancies were 1.678 (0.407 SD - standard deviation), in term pregnancies were 1.034 (0.326 SD) and post term pregnancies were 0.882 (0.104 SD). The cutoff point equal or below 1.1 (likelihood ratio of 12.5). All cases (100%) with 42 or more weeks of gestation showed values under 1.0. Chi square value was 30.9 with p< 0.05.Conclusion

Along the normal pregnancy, ductus arteriosus and pulmonary artery impedance ratio values show opposite tendency patterns. Ductus arteriosus impedance increases towards term as pulmonary impedance decreases. At term, the relative impedance index values are equal or below 1,1 suggesting a circulatory lung hemodynamic changes in accordance with lung maturity. In post-term patients the impedance index values are below 1.0. We recommend the use of this index as a diagnostic tool in the functional assessment of the gestational age.

Image 3,4: Image 3 shows axial plane of the fetal thorax, power Doppler. Pulsed Doppler wave of the pulmonary artery and ductus arteriosus showing the measurement of the acceleration time, ejection time and impedance index of each artery. Image 4 shows relationship between the values of the TA/TE (acceleration and ejection time) and gestational age. Acceleration time of the ductus arteriosus (red lines) and the pulmonary artery (blue lines) with the different periods of lung development by gestational age showing the opposite tendency patterns with crossing point at 37 weeks.

Figure 1,2: Figure 1 shows values of TA/TE (acceleration / ejection time) of the ductus arteriosus (red) and pulmonary artery (yellow). Blue bars demonstrate the values of IRIDAPta/te (impedance relative index between ductus arteriosus and pulmonary artery) showing marked difference between preterm, term and post-term pregnancies. Figure 2 shows distribution of cases according to the cutoff point for both the IRIDAPta/te and gestational age.